1. Field of the Invention
The present invention relates to a cooker, particularly a rice cooker, for use in a microwave (or electronic) oven.
2. Description of the Related Art
FIG. 7 illustrates the relationship between a temperature of water, an amount of water and the ON/OFF state of a current source, when rice is boiled in a conventional electric rice cooker.
In the above FIG. 7, curve A shows the temperature of the water, curve B shows the amount of water remaining in a vessel (i.e., an amount of water corresponding to a remainder obtained by subtracting an amount of water absorbed by the rice from a total amount of water filled in the vessel), and curve C shows the ON/OFF state of the current source of the rice cooker.
As shown in FIG. 7, when a switch of the rice cooker is turned ON, the temperature of the water is gradually raised, the water gradually absorbed by rice, and the water comes to the boil at a point A1. This boiling condition is maintained for a predetermined period of time T (hereinafter the period T is referred to as the boiling stage), then the switch is turned OFF, and the temperature inside the vessel is gradually lowered. Boiled rice is obtained after a post boiling stage (heating stage with heat inertia) and a ripening stage (not shown).
As described above, the conventional rice cooker can produce good boiled rice, because the temperature of the water in the cooker is gradually raised, and thus the necessary amount of water can be absorbed by the rice.
FIG. 6 is a schematical sectional view of a conventional rice cooker for use in a microwave oven.
As shown in FIG. 6, the rice cooker comprises a vessel 1 for holding rice and water, an outside lid 2 for covering the vessel 1, and an inside cover 3 disposed between the vessel 1 and the outside lid 2.
The inside cover 3 is inclined downward to a center portion thereof provided with a hole 3a for returning boiled over components. The hole 3a is provided with escape valve 4, and a silicone rubber packing 5 is disposed to a peripherally around a portion of a bottom of the inside cover 3.
When such a conventional rice cooker is used in the microwave oven, the boiled rice is produced as follows:
(1) Rice and water are introduced into the vessel 1, and then the inside cover 3 and the outside lid 2 are fitted to the vessel 1.
(2) The rice cooker is placed in the microwave oven and heated therein.
FIG. 8 shows the relationship between a temperature of the water, an amount of water, and the ON/OFF state of a switch of the microwave oven. In FIG. 8, curve A' shows the temperature of water in the vessel, curve B' shows an amount of water remaining in the vessel (i.e., an amount of water corresponding to a remainder obtained by subtracting an amount of water absorbed by the rice from the total amount of water filled in the vessel), and curve C' shows the ON/OFF state of the switch of the microwave oven.
As shown in FIG. 8, when the switch of the oven is turned ON, the temperature of the water is raised more rapidly in comparison with the conventional electric rice cooker, but the water is only gradually absorbed by the rice. The water comes to the boil at a point A1'.
Under this condition (i.e., during the boiling stage T'), a pressure within the vessel 1 becomes higher than the atmosphere, and because the vessel 1 is sealed with the packing 5 around the peripheral portion of the inside cover 3, the escape valve 4 is opened and the return hole 3a in the inside cover 3 is blocked by a lower flange 4a. Accordingly, vapor is further, a boiling opaque liquid in the form of a foam is formed out through the aperture 4b. This boiling opaque liquid remains on the inside cover 3, because the return hole 3a is blocked with the lower flange 4a of the escape valve 4.
If the heating is continued at this stage, the contents may boil over from the cooker, and further, the moisture necessary to obtain a properly boiled rice may be lost. Therefore, the microwave oven must be turned OFF.
(3) When the heating is stopped, the pressure in the vessel 1 is reduced, the escape valve 4 is lowered to unblock the return hole 3a in the inside cover 3, and the opaque liquid on the inside cover 3 is returned to the vessel 1.
(4) After the condition (3) above is maintained for a predetermined time (e.g., 10 minutes), the microwave oven is again turned ON to resume heating.
Subsequently, the microwave oven is turned OFF and the boiled rice is obtained after ripening for a sufficient time.
FIG. 9 generally shows a relationship between the temperature of the water and the ON/OFF state of the source switch of the microwave oven, during the above procedures (2) to (4). In FIG. 9, curve A' shows the temperature of the water, and curve C' shows the ON/OFF state of the switch of the microwave oven.
The above mentioned conventional rice cooker for use in the microwave oven has disadvantages such that the packing 5 for sealing the vessel 1 and the peripheral portion of the inside cover 3, and the escape valve 4, must be provided.
Other disadvantages are that the escape valve 4 and the packing 5 must be washed after each cooking, and the cost of producing the cooker is increased because the number of the components thereof is increased.
The conventional rice cooker for use in the microwave oven has further disadvantages, as follows:
(1) Because the heating is carried out by the microwave oven, the temperature A' of water in the vessel 1 is rapidly increased to the boiling point, as shown in FIG. 8, and if the boiling stage is maintained, the moisture necessary to obtain properly boiled rice is lost in the form of vapor or together with the boiled over components. Therefore, the microwave oven must be turned OFF shortly after the contents reach boiling point.
(2) The rate at which the water is absorbed by the rice is increased as the temperature of water is increased, but because the vessel of the conventional rice cooker is composed of a single wall, the temperature in the vessel is rapidly lowered when the microwave oven is turned OFF after the contents reach the boiling point, and as a result, it is difficult to ensure that the rice has absorbed a sufficient amount of water.
Further, the post boiling and ripening treatments cannot be satisfactorily conducted because the temperature in the vessel is rapidly lowered.
Although the opaque liquid collected on the inside cover 3 is returned to the vessel 1 when the microwave oven is turned OFF, the liquid cannot be sufficiently absorbed by the rice, because of the rapid lowering of the temperature in the vessel.
Under the above circumstances, the heating in the conventional rice cooker must be resumed when it is used in the microwave oven as shown in FIG. 9. Namely, as described above, the conventional rice cooker for use in a microwave oven cannot produce good boiled rice unless the microwave oven is operated twice.